Semiconductor technology provides a platform to achieve efficient, compact high power optical sources. High power light emitting semiconductor devices such as light emitting diodes (LEDs) and laser diodes (LDs) are widely used sources of optical power. Although these devices are comparatively efficient, heat generation occurs in a very small volume. For example, high power laser diode bars can generate over 50 Watts of optical power in a volume less than 5 mm3. Electrical to optical conversion efficiency may approach 70% in which case over 3 kW of heat is generated per cubic centimeter. Heat is generally removed by operating the devices on submounts having high electrical and thermal conductivity. The submount, in turn, is placed in direct contact to a cooled heat sink. The submount serves as both an electrical and thermal conduit. Because the contact area of the device is small, the interface between the submount and semiconductor can be a crucial link in the thermal path.
Several patents and publications have been directed to heat removal from semiconductor laser diodes. See, for example, U.S. Pat. Nos. 7,660,335 and 6,865,200, and U.S. Patent Publication No. 2012/0252144. These disclosures facilitate heat removal by placing a thermally and electrically conducting material, i.e., a heat sink, in direct contact to the semiconductor surface. Low thermal and electrical resistance is achieved by soldering the heat sink to the diode with appropriately chosen solder alloys to minimize stress and improve thermal performance. Optical alignment, packaging, and assembly are other considerations addressed by these disclosures and other publications (see, for example, Martin, et al., IEEE J. Quantum Electr. Vol. 28, N. 11, 1992).
The current state of thermal transfer in conventional laser diodes can be further understood from FIGS. 1-6. FIG. 1 is a schematic diagram of an edge emitting laser diode bar 10 with multiple emitters representative of the current state of the art, in accordance with the prior art. The laser diode bar 10 is comprised of semiconductor layers 12 that are typically grown by epitaxy on a single crystal semiconductor substrate 14. The substrate 14 is typically composed of either gallium arsenide (GaAs) or indium phosphide (InP). The laser diode 10 operates when electrical current flows from the p-type contacts 16 to the n-type contact 18. Both p-type and n-type contacts 16, 18 are comprised of multiple metal layers deposited using well established deposition techniques such as sputtering or electron beam evaporation. Thicker metal may be added to one or more of the p-type and n-type contacts 16, 18 by electrochemical deposition to improve lateral heat spreading and to protect the metal/semiconductor interface from possible handling damage (i.e. scratches). The pattern for the p-type contacts 16 shown in FIG. 1 is defined using standard photolithographic processes. FIG. 1 shows individual strips of the p-type contacts 16 on the p-type side of the laser diode 10, which depicts how multiple light emitters for emitting coherent light 20 are integrated in a single chip. However, in practice the metal of the p-type contacts 16 could cover the entire surface of the p-type side of the laser diode 10 and the current flow could be defined by etching a window through an insulating layer, as shown in FIG. 4. Some conventional laser diodes 10 may also isolate emitters using further etch and deposition processes or ion implantation to reduce electrical connectivity in regions of the p-type semiconductor to confine the current.
Laser diode assemblies are used to achieve very high optical power density in a compact form factor. FIG. 2 is a schematic diagram of a laser diode array 30 consisting of stacked laser diode bars 10 from FIG. 1 connected in series by thermally and electrically conducting spacers, in accordance with the prior art. The laser diode array 30 has diode bars 10 that are stacked vertically and separated by spacers 32. Electrical current flows from the positive contact 34 to the negative contact 36. The laser light (20 in FIG. 1) is emitted out the front facets 38 of the diode bars 10. Additional optical components that are not shown may be used to collect, focus, and/or collimate the light. This configuration requires that the electrical resistance be as low as possible since resistive loss generates excess heat. Therefore, it is important that the spacers 32 exhibit good electrical conductivity. Additionally, the spacers 32 conduct heat from the laser diode bars 10 to the cooled heat sink 40 so they must also exhibit high thermal conductivity. Heat is generated inside the laser diode bars 10 near the active region so the heat transfer through the p-contact(s) (16 in FIG. 1) can play a crucial role in device performance and reliability.
FIG. 3 is a detailed schematic diagram of epitaxial semiconductor layers 12 that comprise laser diode bars 10 of FIG. 1, in accordance with the prior art. The vertical waveguide for the laser diode 10 is formed by growing waveguide layers 50, 52 that have larger refractive index than the surrounding p-type cladding layer 54 and n-type cladding layer 56. The electrical current injected from the cladding layers 54, 56 is converted to coherent optical power in the active layer 58 positioned between the waveguide layers 50, 52.
FIG. 4 is a schematic diagram of a single laser diode emitter depicting epitaxial layers in the laser diode along with the contact and features defining the lateral waveguide, in accordance with the prior art. With reference to FIGS. 3-4, lasing operation requires that light generated in the active region 58 be guided in both the vertical and lateral dimensions. Lateral guiding can be achieved by limiting the lateral extent of the current flow as indicated in FIG. 4 by etching a mesa 60 into the semiconductor. Coherent light 20 is emitted from the laser diode 10 on a facet plane (illustrated perpendicular to the cross-section shown in FIGS. 3-4; 16 in FIG. 2). Emission intensity is controlled by controlling the reflectivity at the facet by means of mirror coating layers. Laser light is primarily emitted from the facet with low reflectivity.
Resistive loss is an important heat source in the laser diode. The substrate 14 is typically polished after growth of the epitaxial layers, thereby reducing the thickness on the n-side, which also reduces the electrical and thermal resistance between the active region 58 and the n-side contact 18. The final thickness after polishing is limited by the requirement that the laser diode 10 retain mechanical stability for subsequent handling. The electrical resistivity of the p-type semiconductor material is generally higher than that of the n-type material. Therefore, the diode 10 is designed so the p-type cladding layer 54 and the p-type cap layer 62 are as thin as possible, on the order of 1-2 μm. The p-type clad layer 54 must be thick enough to guide the coherent light in the waveguide layers 50, 52, 58 but thin enough to minimize series resistance on the p-side of the diode 10. The thermal resistance on the p-side of the diode 10 can also be improved if the p-type layers 54, 62 are as thin as possible. Electrical resistance at the p-type contact can be further reduced by growing a thin, highly doped cap layer 62 on the p-type cladding layer 54 to which the metal contact 16 is contacted. When possible, the cap layer 62 is chosen to have a small bandgap which reduces the electrical Schottky barrier to the metal contact 16. The p-type resistivity, contact to the metal contact 16 and the reduction of the Schottky barrier height at the p-type contact 16 are important considerations to minimize heating on the p-side of the laser diode 10.
FIG. 4 further shows the cross-section of the edge emitting laser diode 10 in which the lateral waveguide is defined by an etched mesa 60 using standard wet chemical processing and photolithography. An electrically insulating layer 64 helps confine the current to the lateral waveguide, thereby improving electrical to optical conversion efficiency. The mesa structure 60 also guides the optical power 20 which facilitates coupling of the emitted light at the output facet and improves electrical to optical conversion efficiency. The insulating layer 64 typically has lower thermal conductivity than the semiconductor 12 and metal contact 16 so heat flows predominantly through the p-type contact 16. Heat transport through the interface between the p-type contact 16 and p-type semiconductor 12 is consequently of great importance for device performance.
FIG. 5 is a schematic diagram of a laser diode bar 10 with the p-side mounted to a thermally and electrically conducting spacer 70, in accordance with the prior art. Multiple waveguides 12 defined laterally along the width of the laser diode emit light 20 that is transmitted from the low-reflectivity laser facet 38. Heat removal from the laser diode 10 through the p-type contact 16 is facilitated by minimizing the distance from the laser diode active 12 region to the p-type contact 16 and spacer 70.
The thermal barrier at the p-type contact 16 poses a fundamental limit on the removal of heat through the p-type contact 16. FIG. 6 depicts a corresponding temperature profile at the p-type contact 16 with the thermal barrier, in accordance with the prior art, and as noted by Rieprich, et al. (Proc. SPIE, Vol. 10085, N. 1008502, 2017). As shown in FIG. 6, the temperature in the semiconductor 12 changes linearly and may even increase at the portion of the semiconductor 12 positioned near or approaching the p-type contact 16 due to significant heat generation occurring in the p-type semiconductor 12. An abrupt temperature change occurs at the interface 80 of the p-type contact 16 with the metal spacer 70 due to the thermal barrier, such that the temperature decreases in the metal spacer 70 away from the interface 80.
Rieprich, et al. recently conducted a study which presented results from an investigation into thermal lensing in broad area edge emitting laser diodes. Thermal lensing occurs when the local refractive index in the laser diode waveguide increases due to heating. Multiple negative effects, including beam quality degradation, are attributed to thermal lensing. The authors showed that a thermal conductance barrier between the p-type semiconductor and metal contact, e.g., the barrier at interface 80 in FIG. 6, places a lower limit on thermal resistance. The thermal barrier was attributed to the fact that the heat transport mechanism in the semiconductor is different than that in the metal. The thermal barrier at interfaces between metals and non-metals was also described in a doctoral thesis entitled, “Thermal Boundary Conductance between metals and dielectrics” (École Polytechnique Fédérale De Lausanne, Faculté Des Sciences Et Techniques De L'Ingeniuer, 2013). Heat transport in metals is dominated by electron transport while heat transport in non-metals is primarily due to phonon dispersion. The barrier is attributed to the transfer of thermal energy from one mechanism to the other. Rieprich et al. observed that the thermal barrier was common to multiple laser diodes from various suppliers. The thermal barrier was consistently in the range 0.12-0.19 Kelvin per mm2 which is consistent with measurements performed by the inventors of this disclosure.
Thus, with this understanding of the present state of heat removal in conventional semiconductors, it is evident that improved heat removal and improved heat flow from semiconductors to a heat sink is needed to address the aforementioned deficiencies and inadequacies.